1. Field of the Invention
The present invention relates to an apparatus for measuring a form of a surface. More particularly, the present invention relates to a measuring apparatus that operates to measure a form of a surface of an object according to the light reflected off the surface of the object.
2. Description of the Prior Art
With miniaturization and sophistication of electronic instruments such as, for example, a camcoder and a wide screen TV set, attention has been paid to the high-density mounting of electronic components. Accordingly, it is likely that more precise measurement of the thickness of cream solder coated on a circuit board or the inspection for a state of mounted components will be required.
For example, in order to inspect the thickness of a solder layer immediately after the cream solder is coated, a laser diode or a Hexe2x80x94Ne laser operates to radiate a laser beam having a wavelength of 630 to 810 nm onto an object on which the solder is coated and to sense the beam reflected from the solder-coated object using an image sensor or a location sensor which employs a CCD (Charged Coupled Device).
This type of apparatus for measuring the form of a surface such as the apparatus for measuring a thickness of solder has been conventionally arranged as shown in FIG. 1, for example. The apparatus 50 for measuring a surface form operates as follows. A laser source 51 emitts a diffused laser beam having a wavelength of 632.8 to 830 nm. The diffused laser beam passes through a collimator lens 52 so that the laser beam is transformed into a parallel laser beam. The parallel laser beam passes through a concave cylindrical lens 53 and a convex cylindrical lens 54 so that the parallel laser beam is linearly radiated onto a surface 55 of an object to be measured while keeping the beam perpendicular to the surface 55. The beam is reflected on the surface 55 obliquely to an optical axis of an illuminating optical system comprised of a collimator lens 52, a concave cylindrical lens 53, and a convex cylindrical lens 54. The reflected beam is incident on a CCD image sensor 57 at right angles through an imaging lens 56. The CCD image sensor 57 corresponds to a two-dimensional image sensor. The CCD image sensor 57 receives the reflected beam and senses a form of emission lines appearing according to the form of the surface 55.
The data about the sensed form is read by a computer dedicated to processing graphics. This type of computer uses a triangulation survey at one point to be measured and two measuring points located differently so that the computer can determine the form of the surface of the object to be measured from the coordinates of the point to be measured, a distance between the measuring points, and a visual angle.
For sensing the form of emission lines appearing according to the form of the surface of the object to be measured, the apparatus 50 is arranged to locate the CCD image sensor 57 perpendicularly to the optical axis of the reflected beam and obliquely to the optical axis of the laser beam radiated onto the object 55.
On the CCD image sensor 57, a light section image is magnified too much by bringing about a change of width of the laser beam resulting from variable heights of the object surface 55 spaced from a datum level, that is, the variable thicknesses of the surface 55 in concert with a defocusing resulting from images on the sensor 57 located differently from each other. In the apparatus 50, therefore, the signal intensity of the light section image is so variable on the object surface according to the variety of the surface thickness. This variety makes the S/N ratio worse and the measuring resolution lower, which become the obstacle to precise operation, thereby making it difficult to exactly measure the form of the object surface.
Further, in case that the apparatus 50 operates to measure a thickness of the coat of the cream solder, the apparatus 50 cannot offer the highly reliable measurement of the object surface, since a resist plane adjacent for the solder coat is set as the datum level.
This is because the laser beam having a wavelength of 632.8 to 830 nm emitted from the laser source 51 has a greatly different reflectance for the resist component than the reflectance of the solder. FIG. 2 is a graph showing a spectral characteristic for reflectance with respect to the resist component. The laser beam having the above-mentioned wavelength has a reflectance of 0.5% or less for the resist component, while the laser beam has a reflectance of about 70% for the solder portion. The latter is 100 or more times as large as the former. Hence, if a video signal in the solder portion is adjusted not to be saturated, the difference in video signal intensity becomes too much large between the resist portion and the solder portion. This difference makes the S/N ratio worse, which inhibits the precise sensing of video signals in the resist portion, thereby making it difficult to measure the object surface with high reliance.
If the laser beam is condensed linearly, the line width of the resist portion is greatly different from that of the solder portion. This makes a measuring accuracy worse.
According to an aspect of the invention, an apparatus for measuring a surface form includes an emitting optical system for radiating a beam linearly from a light source onto a surface of an object to be measured, an observing system having a optical axis directed differently from the optical axis of the emitting optical system, and a two-dimensional image sensor located to be in conjugation with the height direction of the object to be measured, that is, the direction of the optical axis of the emitting optical system, for observing a light section image formed by the linear beam radiated onto the surface of the object to be measured. Hence, this measuring apparatus suppresses defocusing even at a location spaced from the center of the image and lower a line width of the light section image and change of a light intensity as compared with the prior art, thereby keeping the measurement more exact with no degrade of the S/N ratio. Further, since the reflectance against the resist portion is made larger as compared with the prior art, the video signal of the resist portion is made larger so that the distribution of the intensity may be sensed at a high S/N ratio. This makes the measurement of the surface form more reliable and it possible to exactly measure the surface form of a mounting substrate. Moreover, unlike the prior art, the measuring apparatus uses a visible laser beam. Hence, the apparatus does not need a reference light, which results in decreasing the number of parts and reducing the overall apparatus in size.
According to another aspect of the invention, an apparatus for measuring a surface form includes an emitting optical system for obliquely radiating the linear beam from a light source onto the surface of the object to be measured and an observing system having a two-dimensional image sensor located perpendicularly to the surface of the object to be measured, for observing a light section image formed by the linear beam applied onto the object surface. As compared with the prior art, the line width of the light section image and the change of the intensity distribution are far less variable. This makes it possible to keep the S/N ratio high in sensing the image, thereby enhancing the reliance of measuring the surface form and being able to more exactly measure the surface form of the mounting substrate.